1. Field of the Invention
The present invention relates to a production process of a light amount adjustment member used in optical instruments such as digital cameras and video cameras, a light amount adjustment member, a light amount adjustment device, and a photographing apparatus.
2. Related Background Art
In an image forming (photographing) optical system used in an optical instrument, such as a camera, a light amount adjustment device that controls light quantity of an incident beam, i.e., the so-called diaphragm device, is generally contained. In such a diaphragm device, a plurality of diaphragm blades form an opening of a prescribed area, and the quantity of a beam passing through the opening is controlled by controlling an opening diameter of the opening by an actuator. However, an influence of diffraction caused at an end of the diaphragm blade becomes great as the opening diameter of the opening is made small, so that the image forming performance of the image forming optical system is deteriorated. On the other hand, there has been known a technique that a filter as a light amount adjustment member is provided at a part of the diaphragm blade in order to avoid this defect, thereby attenuating the quantity (light quantity) of a bundle of rays passing through the opening by the optical filter instead of making the opening diameter small. The optical filter used for such purposes is required to lessen optical defects such as light scattering, refraction abnormality and spectral transmittance deviation.
Those of the type in which a light-absorbing coloring material such as a pigment or dye is mixed and incorporated into a transparent film-forming material have heretofore been generally used for such a light amount adjustment member. However, light amount adjustment members produced by this method have been very expensive and have not sufficiently satisfied cost reduction required of increasing demand. In addition, with the method in which the coloring material is incorporated into the transparent film-forming material, it is markedly difficult to produce a light amount adjustment member having a continuous or stepwise density distribution (hereinafter referred to as “multi-density”).
As another production method, for example, Japanese Patent Application Laid-Open No. H5-173004 discloses a method of producing a multi-density light amount adjustment member using a silver halide film. However, the light amount adjustment member obtained by this method causes a problem characteristic of the use of the silver halide film that the rectilinearity of rays passed through a filter is impaired by reflection of a beam on the surfaces of silver particles contained in the filter and diffraction of a beam passed through ends of the silver particles to deteriorate the image forming performance of the optical system.
Japanese Patent Application Laid-Open No. H10-133254 discloses a method of producing a multi-density light amount adjustment member by a vapor deposition method. However, this method incurs increase in production cost and is thus expensive. In addition, since the film thickness thereof varies according to the density, a problem arises in that a difference in film thickness arises between a high-density portion and a low-density portion, and consequently, an optical path difference arises to deteriorate resolution. Further, according to this method, a light amount adjustment member in which the density distribution varies stepwise can be produced, but that having a continuously varied density distribution is difficult to produce.
Japanese Patent Application Laid-Open No. H10-96971 discloses a process of producing a light amount adjustment member having a density distribution by first incorporating organic coloring matter, which fades by light, in a film forming material, and partially irradiating the resultant film with high-energy light to decompose the organic coloring matter at irradiated portions. However, according to this process, usable coloring materials are limited to those which fade by light. It is therefore very difficult to obtain a product having sufficient optical properties. In addition, it may be easily inferred that resulting products will become very expensive because of the production process.
Further, Japanese Patent Application Laid-Open No. 2000-352736 discloses a process of producing a filter as a light amount adjustment member that a transmittance varies steplessly by forming a film having a single density in the form of halftone dots by vapor deposition or a printing process such as a photomechanical process and varying a halftone dot pattern from position to position. However, in such a process, a film of a prescribed density is formed by the photomechanical process or vapor deposition. Even if either process is adopted, a problem that the apparatus becomes large and expensive to increase the production cost of the filter arises.